The loss of synaptic connections correlates with cognitive decline in many neurodegenerative disorders, including HIV-1 associated dementia (HAD). An assay was developed to image intact postsynaptic densities (PSDs) based on detection of clusters of the scaffolding protein PSD95 fused to green fluorescent protein (PSD95-GFP). In hippocampal neurons grown in primary culture, PSD95-GFP puncta were lost following exposure to factors released by HIV-1 infected cells. PSD loss was reversible. Cannabinoids, drugs given to AIDS patients clinically and widely used illicitly, modulate synapse loss. Three specific aims examine the effects of cannabinoids on the loss and recovery of synapses following exposure to HIV-1 proteins. 1) The hypothesis that neurotoxin-induced synapse loss is a mechanism to reduce excessive glutamatergic stimulation will be tested. These studies will delineate the signaling pathways that lead to synapse loss and cell death providing a foundation from which to study how drugs of abuse affect the balance between synaptic function and neuronal survival. 2) The hypotheses that recovery from synapse loss requires changes in NMDA receptor function and is guided by the location of pre-existing presynaptic terminals will be tested. By determining the mechanisms that initiate and direct the recovery of synapses, these studies will identify sites where cannabinoids might affect the ability of neurons to integrate back into the synaptic network. 3) The effects of acute and chronic exposure to cannabinoids on synaptic changes induced by HIV-1 proteins will be determined. The hypothesis that the mechanism by which cannabinoids modulate synapses depends on the toxic stimulus and the duration of drug treatment will be tested. These studies will provide insight into the processes that underlie cognitive decline in HAD and will enable us to determine the influence of cannabinoids on the balance between network function and cell survival. This project will provide a foundation to guide the development of drugs to improve function in HAD patients and will identify sites where drugs of abuse might interact with the formation and loss of synapses.